Knowledge of the behavior of materials under extreme conditions is valuable in many fields of science, including materials engineering, energy and defense technologies, reaction chemistry, and environmental and planetary sciences. However, currently available experimental tools for high-pressure static experiments in diamond anvils cells in a vast majority of cases provide only information about time-averaged properties, and dynamical information about ultrafast phenomena at the microscopic level is missing. The goal of the work is to develop a new instrument which combines conditions of high static pressure with dynamic loading (e.g., by shock and/or thermal waves) and probes materials properties in situ under these conditions using ultrafast (femto- to picosecond) pulsed laser interferometric and spectroscopic techniques with the appropriate microscopic spatial resolution. With the development of this instrumentation, studies of materials in the diamond anvil cell will be extended to temperatures beyond 10,000 K at pressures beyond 100 GPa. Moreover, observations of physical and chemical phenomena on ultra-short time scales (40 fs to ps), comparable to times of fast phase transformations and elementary chemical reactions, will be enabled. This new approach has the potential to reveal the formation of new materials with unique properties and lead to the discovery of new phase transformations and chemical reactivity, as well as advancing our understanding of the Earth and planetary interiors. The instrument will become available for collaborators, and to independent users from NSF-supported programs such as COMPRES and the Carnegie Summer Intern Program, as well as from the DOE-supported CDAC high-pressure center, headquartered at Carnegie. Area high school students, undergraduates and graduate students, including those from local historically-black Howard University with whom Carnegie has an active collaboration, will benefit from the scientific training provided by participation in the development of the proposed instrument and the scientific studies conducted with it. Layman summary: Studies of materials under extreme conditions (high pressure and temperature) can be useful in the search for new materials and for better understanding the compositions and processes in the Earth and other planets. However, many of these studies currently lack the experimental speed to study very fast phenomena on the level of atoms and electrons, such as chemical reactions and transformations to new material phases. This project is aimed at developing a new instrument which would create extreme conditions through rapid heating and compression with bright laser pulses (1 millionth of 1 millionth of a second in length or less), and measure materials physical and chemical properties under these rapidly-changing conditions. This instrument has the potential to observe new behaviors in materials such as rapid changes in structure and chemical composition on the natural time scales of these phenomena. Due to the unique nature of the conditions available for this new instrument, new materials and chemical compounds with extraordinary properties can be discovered, and advanced understanding of the Earth and planetary interiors will be achieved. The proposed instrument will become available for collaborators, and to independent users from NSF-supported programs such as COMPRES and the Carnegie Summer Intern Program, as well as from the DOE-supported CDAC high-pressure center, headquartered at Carnegie. Area high school students, undergraduates and graduate students, including those from local historically-black Howard University with whom Carnegie has an active collaboration, will benefit from the scientific training provided by participation in the development of the proposed instrument and the scientific studies conducted with it.
The major goal of this proposal is to develop new techniques for generating extreme conditions of high pressure, temperature, and strain rate and also in situ, time-resolved diagnostics of materials subjected to these conditions. The extreme conditions will be created by applying pulsed laser heating and laser driven dynamic compression to materials confined under high static pressures in diamond anvil cells (DAC). The material state will be interrogated by ultafast interferometric and optical probes -Coherent Antistokes Raman Spectroscopy (CARS) and broadband optical spectroscopy (BBOS). We have developed two ultrafast optical spectroscopy techniques, which utilize a super-continuum and double-frequency outputs of a ps 2 microJ 1060 nm laser: BBOS and CARS. These techniques use a super-continuum broadband output created in a Photonic Crystal Fiber (PCF) [1]. Moreover, we continue working on setting up similar systems BBOS (including in infrared) and CARS, which would use the purchased on MRI funds ultrafast amplified Ti: Sapphire laser system. Using a pulsed fiber heating laser, super-continuum broadband pulse created in PCF, and continuous 532 nm laser, we have set up a system for measuring optical properties of materials at very high temperatures and pressures. Both, repetitive and single-shot modes of operation are available using ICCD (in time domain) and/or a streak-camera (purchased using the MRI funds) (in the frequency domain). In the latter mode, we have used a split window configuration, which allowed detecting the emission and the optical spectra simultaneously, but in different spectral windows. We have performed several pilot experiments on optical properties of Xe, Ar, Ne, H2, O2[1], N2, MgO, H2O precompressed in the DAC up to 150 GPa and pulsed laser heated up to 10,000 K. We find that unlike the previous anticipations N2, Ar [2], Ne, and H2 show semiconducting behavior at high temperatures (Fig. 1). Study of hydrogen under extreme conditions is one of the major scientific drivers of this project. In collaboration with the University of Edinburgh group (E. Gregoryanz), we have studied Raman spectra and optical properties of hydrogen to 318 GPa at 300 K [3]- conditions which were unattainable until quite recently. The results show that at 240 GPa hydrogen transforms to a new phase IV, which consists of a mixture of graphene-like layers, formed of elongated H2 dimers experiencing large pairing fluctuations, and unbound H2 molecules. The optical properties of hydrogen above 270 GPa are consistent with its semimetallic electronic band [3, 4]. Recently we have completed a study of H2-D2 mixtures, which reveal a novel Anderson localization of vibron modes in phase IV [5]. In collaboration with the LLNL group (M. Armstrong), we have explored a new path for achieving very high compression by performing laser driven compression on picosecond time scale [6]. For dynamic compression, the volume of compressed material varies monotonically with the time scale of compression. Thus, for sub-100 ps compression, the compressed volume and compression energy can be many orders of magnitude smaller than nanosecond scale experiments which achieve comparable absolute increases in density. We find that quasi-isentropic compression of precompressed up to 25 GPa deuterium can be performed in less than 100 picoseconds, at least one order of magnitude shorter than previous work [7] (Fig. 2). These results suggest a new path to obtaining hydrogen at high density using low energy, table top laser systems. The technical developments of this work enabled measurement of the lattice and radiative thermal conductivity of the deep Earth’s materials, knowledge of which are important for understanding of the Earth thermal history and dynamics. A number of postdoctoral associates working on the projects have been trained in novel ultrafast laser spectroscopy techniques and their applications to geo- planetary and material sciences. 1. A.F. Goncharov et al., Development of Ultrafast Spectroscopic Techniques to Study Rapid Chemical and Physical Changes in Molecules under Extreme Pressure and Temperature Conditions, Mater. Res. Soc. Symp. Proc., 1405 (2012). 2. R.S. McWilliams, D.A. Dalton, M.F. Mahmood, A.F. Goncharov, Optical properties of warm dense argon in the laser-heated diamond anvil cell, Phys Rev B, in press (2013). 3. R. T. Howie et al., Mixed Molecular and Atomic Phase of Dense Hydrogen, Phys. Rev. Lett. 108, (2012) 125501. 4. A.F. Goncharov et al., Phys Rev B 87 (2013) 024101. 5. R. T. Howie, I. B. Magdau, G. J. Ackland, E. Gregoryanz, A. F. Goncharov, Phonon Localisation in Dense Hydrogen-Deuterium Binary Alloy, submitted. 6. M. R. Armstrong, et al., Appl. Phys. Lett. 101, 101904 (2012) 7. M. R. Armstrong, J. C. Crowhurst, S. Bastea, J. M. Zaug, A. F. Goncharov, Quasi-isentropic compression of deuterium on an ultrafast time scale, Phys Rev. Lett. Under review
